Iron carbide is an advantageous feed component in the production of steel which avoids many of the environmental and economical drawbacks associated with conventional blast furnace and coke oven steel making processes. A charge of iron carbide and scrap can be converted directly to steel in a basic oxygen furnace, a ladle furnace, an electric arc furnace or the like, providing significant environmental and economic advantages, especially when the price of high quality scrap is high. Accordingly, the ability to economically produce large amounts of high quality iron carbide is extremely desirable.
A basic process of producing iron carbide by the reduction of iron ore involves the use of a fluidized bed reactor in which an iron oxide containing feed stock, typically comprising magnetite (Fe.sub.3 O.sub.4) and hematite (Fe.sub.2 O.sub.3), is reduced to iron carbide by action of a reactant gas. The reactant gas includes a reducing gas, which is typically hydrogen, and a carbon bearing gas which is typically methane. In the fluidized bed reactor the reaction results in a five component gas system in which hydrogen (H.sub.2), methane (CH.sub.4), carbon dioxide (CO.sub.2), carbon monoxide (CO), and water (H.sub.2 O) exist in equilibrium. Inert gases such as nitrogen are also frequently present in the system. Attempts have been made to improve the production rate and product quality of iron carbide by controlling the reaction parameters such as temperature and pressure to effect the equilibrium gas composition in the reactor. However, there has been little success in obtaining improved results on a commercial scale.
Attempts have been made to improve the basic iron carbide process by preheating the feed stock in an oxidizing atmosphere. In the typical iron carbide process it is necessary to preheat the reactor feed in order to drive off water, which is believed to hinder the reaction, and to heat the feed to a suitable temperature. This is typically done in one or more preheating kilns or preheating cyclones. Efforts have been made to improve the preheating step by oxidizing any magnetite in the feed stock to hematite, which is believed to convert more readily to iron carbide. Although it is generally agreed that hematite has a crystalline structure conducive to the reaction, the oxidative preheat also increases the hydrogen demand in the reactor. This can be a significant limitation in a large scale production facility.
In direct reduction processes, there have also been attempts to use more than one reactor in series. Although this provides the advantages of reduced backflow and reduced likelihood of unreacted feed stock making it through the reactor, these processes have employed so called counter current gas flow, wherein the same gas and reaction conditions are used in both beds. This concept has been adapted to a single fluidized bed by the use of baffles. By approximating a plug flow reactor through channels formed by a series of baffles that prevent back-flow and intermixing between channels, one in essence creates a series of individual reactors within a single fluidized bed apparatus.
Although the foregoing processes will produce iron carbide, there is still a need to improve the process in order to make it efficient and economically advantageous on a commercial scale. In particular, there is a need to increase the efficiency, rate and throughput of the process, while minimizing the costs and energy demands so that the large scale production of iron carbide can be successfully implemented in a commercially viable manner.